Document the contract functions the client does not cover

[Abioladory123]

Summary

The README's client section lists the methods the SDK provides. It does not say
which contract functions have no wrapper, so a consumer discovers the gap by
looking for a method that is not there and then wondering whether they have
misread the docs.

Stating the coverage plainly turns an absence into a known limitation, and gives
a contributor an obvious list of things to add.

Acceptance criteria

• The README lists every contract function and whether the client wraps it.
• Unwrapped functions link to the issues proposing them.
• The list is placed next to the client API reference so it is hard to miss.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/novatip-sdk.git
cd novatip-sdk
git remote add upstream https://github.com/Novatip/novatip-sdk.git
npm install

Create a branch for this issue:

git checkout -b docs/client-coverage

Suggested commit message:

docs: document unwrapped contract functions

Run npm run lint, npm run typecheck, and npm run build before pushing, then open a pull request from your fork against main and link this issue. See the contributing guide for the full workflow.